Electro-optical device and method for manufacturing the same

ABSTRACT

An electro-optical device and a method for manufacturing the same are disclosed. The device comprises a pair of substrates and an electro-optical modulating layer (e.g. a liquid crystal layer having sandwiched therebetween, said pair of substrates consisting of a first substrate having provided thereon a plurality of gate wires, a plurality of source (drain) wires, and a pixel matrix comprising thin film transistors, and a second substrate facing the first substrate, wherein, among the peripheral circuits having established on the first substrate and being connected to the matrix wirings for the X direction and the Y direction, only a part of said peripheral circuits is constructed from thin film semiconductor devices fabricated by the same process utilized for an active device, and the rest of the peripheral circuits is constructed from semiconductor chips. The liquid crystal display device according to the present invention is characterized by that the peripheral circuits are not wholly fabricated into thin film transistors, but only those portions having a simple device structure, or those composed of a small number of devices, or those comprising an IC not easily available commercially, or those comprising an expensive integrated circuit, are fabricated by thin film transistors. According to the present invention, an electro-optical device is provided at an increased production yield with a reduced production cost.

This application is a Divisional of application Ser. No. 08/962,600filed Oct. 31, 1997 now U.S. Pat. No. 6,306,213; which itself is aDivisional of Ser. No. 08/231,644 filed Apr. 22, 1994, now U.S. Pat. No.5,849,601; which itself is a Continuation of Ser. No. 08/217,211 filedMar. 24, 1994, abandoned; which is a Continuation of Ser. No. 07/811,063filed Feb. 20, 1991, abandoned; which is a Continuation-in-Part of Ser.No. 08/293,201 filed Aug. 19, 1994, now U.S. Pat. No. 5,614,732; whichis a Continuation of Ser. No. 07/967,564 filed Oct. 28, 1992, abandoned;which is a Continuation of Ser. No. 07/673,821 filed Mar. 21, 1991,abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electro-optical device whichcomprises thin film transistors (referred to hereinafter as TFTs) and toa method for manufacturing such a device.

2. Description of the Prior Art

More attention is now paid to flat displays than to CRTs (cathode raytubes) for use in office automation (OA) machines and the like, andparticularly, there is an increasing demand for large-area displaydevices. There is also an active and rapid development in theapplication field of flat displays such as wall television (TV) sets.Furthermore, there are demands for color flat displays and finer displayimages.

A liquid crystal display device is known as a representative example ofa flat display. A liquid crystal display device comprises a pair ofglass substrates having incorporated therebetween a liquid crystalcomposition together with electrodes, and the images are displayed bythe change of state of the liquid crystal composition upon applicationof an electric field thereto. The liquid crystal cells may be driven bythe use of a TFT or other switching devices, or by making it into asimple matrix structure. In any case, a driver circuit is established atthe periphery of a display to supply signals for driving liquid crystalsto column lines (extending in Y direction) and row lines (extending in Xdirection).

The driver circuit is generally composed of a single crystal silicon MOSintegrated circuit (IC). The IC is provided with pad electrodes, eachcorresponding to each of the display electrodes, and a printed circuitboard is incorporated between the pad and the display electrodes toconnect first the pad electrode of the IC with the printed circuit boardand then the printed circuit board with the display. The printed circuitboard, which in general is made of an insulator board made frommaterials such as a glass fiber-reinforced epoxy resin or a paper filledepoxy resin, or of a flexible plastic board, requires an area equivalentto or even larger than the display area. Similarly, the volume thereofshould have to be made considerably large.

Thus, because of the construction as described hereinabove, aconventional display suffers problems as follows:

-   -   (1) A superfine display cannot be realized. Since each of the        wirings of the display electrodes for the X direction and the Y        direction of the matrix, or the source (drain) wirings or the        gate wirings should be connected individually to each of the        contacts on the printed board, the minimum distance between the        connecting portions technologically achievable by the up-to-date        surface mounting technology is limited to a certain length;    -   (2) A display device as a whole occupies a large area and        volume. A display device comprises indispensable parts in        addition to the display itself, inclusive of the printed circuit        board, the ICs, and the connecting wirings, which require an        area and volume about several times as large as those of the        display alone; and    -   (3) The connections are of low reliability. Quite a large number        of connections should be established between the main display        and the printed circuit board, as well as between the printed        circuit board and the ICs; moreover, not a small weight is        casted on the connecting portions.

As a means to overcome the foregoing problems, there is proposed,particularly in a display device comprising an active matrix as theswitching device, to construct a display device comprising the activedevice and the peripheral circuits on a same substrate using TFTs ratherthan semiconductor chips. Such a construction indeed solves the threeproblems mentioned hereinabove, however, it newly develops problems asfollows:

-   -   (4) The production yield of the display is low. Since the        peripheral circuits also are made from TFTs, the number of the        devices to be fabricated on the same substrate is increased and        hence the production yield of the TFT is lowered;    -   (5) The production cost is increased. Since the peripheral        circuit portion comprises a very complicated device structure,        the circuit pattern accordingly becomes complicated and hence        the cost increases. Furthermore, with the increase in the        multilayered wiring portion, an increase in process steps as        well as a decrease in the production yield of the TFTs occur;        and    -   (6) It requires a treatment at a high temperature and hence the        use of an expensive quartz substrate becomes requisite. Since a        quick response is required to the transistors which constitute        the peripheral circuit, the semiconductor layer should be        treated at a high temperature to obtain a polycrystalline layer        to be used as the transistors.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the six problemsmentioned above by taking balance among them and to provide a moreeconomical electro-optical device at a high yield.

This and other objects of the present invention have been attained by anelectro-optical device comprising a pair of substrates and anelectro-optical modulating layer having sandwiched therebetween, saidpair of substrates consisting of a first substrate having providedthereon thin film transistors and matrix lines comprising a plurality ofgate wires (extending e.g. in Y direction) and a plurality of source(drain) wires (extending e.g. in X direction), and a second substratefacing the first substrate, wherein, among the peripheral circuits beingconnected to the matrix lines, only one direction side of saidperipheral circuits is constructed from thin film semiconductor devicesfabricated by the same process utilized for the thin film transistors,and the rest of the peripheral circuits is constructed fromsemiconductor chips. The electro-optical modulating layer may comprise aliquid crystal.

In the electro-optical device according to the present invention, theperipheral circuit only on one side is fabricated as TFTs. In such amanner, a general use economical ICs can be utilized on one side, whilefabricating the side comprising peripheral circuits which are notcommercially available or which are costly, as TFTs.

Furthermore, by making only one side into TFTs, the number of the TFTsin the peripheral circuit portion can be considerably reduced. Forexample, the number of the TFTs can be halved in case the peripheralcircuits for the X and the Y directions have the same function. Thus,the production yield of a TFT on the same substrate is increased by twotimes in a simple calculation.

According to another embodiment of the present invention, there isprovided an electro-optical device comprising a first substrate havingprovided thereon a pixel matrix furnished with complementary TFTs andmatrix lines comprising a plurality of gate wires (extending e.g. in Ydirection) and a plurality of source (drain) wires (extending e.g. in Xdirection), said first substrate making a pair with a second substratefaced thereto and an electro-optical modulating layer having supportedbetween the pair of substrates, characterized by that at least a part ofthe peripheral circuits being connected to the matrix lines isfabricated into complementary TFTs similar in structure of the activematrix (e.g. the complementary TFTs of the pixel matrix) and beingfabricated also in a same process as that of the active matrix, and thatthe rest of the peripheral circuits are constructed by semiconductorchips, said peripheral circuits being connected to the X or Y directionmatrix wires having established on the first substrate. Theelectro-optical modulating layer may comprise a liquid crystal.

More precisely, the electro-optical device according to the presentinvention is characterized by that the peripheral circuit is not whollyfabricated into TFTs, but only those portions having a simple devicestructure, or those composed of a small number of devices, or thosecomprising an IC not easily available commercially, or those comprisingan expensive IC, are fabricated as TFTs. It is therefore an object ofthe present invention to increase the production yield and to reduce theproduction cost of an electro-optical device.

Accordingly, by fabricating a part of the peripheral circuits as TFTs,the number of external ICs is reduced and therefore the production costis reduced.

Furthermore, by fabricating the peripheral circuits and the activematrix in the same process to give complementary TFTs (CTFTs), the pixeldriving capacity is increased and a redundancy is imparted to theperipheral circuit. Hence it is possible to drive the electro-opticaldevice with a greater allowance.

If the peripheral circuits were to be wholly fabricated into TFTs, thedevice requires extension of the display substrate along both of the Xand Y directions. This inevitably leads to an unfavorable increase inthe total occupancy area. In contrast, if only a part of the peripheralcircuits were to be fabricated into TFTs, it results in a slightincrease of the substrate which can be readily accommodated to the outerdimension of the computers and other apparatuses to which theelectro-optical device is assembled. Thus, there is provided anelectro-optical device having a small occupancy area and volume.

A high technology is required to fabricate the complicated portions inthe peripheral circuit as TFTs. For example, a device structure whichrequires a multilayered wiring, or a portion having a function as anamplifier can be mentioned as those particular portions. However, if theperipheral circuit were to be partially made into TFTs by replacing onlythe simple device structures and portions of simple function with TFTswhile using the conventional ICs for the portions in which hightechnology is required, an electro-optical device can be realized atreduced cost with a high yield.

Furthermore, a partial adoption of TFTs considerably reduces the numberof TFTs within the peripheral circuit portion. In a simple calculation,it can be seen that the number of the TFTs can be reduced to a half ifthe function of the TFTs for the X direction and the Y direction werethe same. It is possible, by reducing the number of devices to befabricated as TFTs, to increase the production yield of the substrate.In addition, there is provided a low cost electro-optical device reducedboth in the area of the substrate and in volume.

In an electro-optical device according to an embodiment of the presentinvention, a single pixel may be constructed by connecting two or moreCTFTs with a pixel. Otherwise, a single pixel may be divided into two ormore pixels, and to each of the divided pixels may be connected one ormore CTFTs.

The ICs of the residual peripheral circuit portions which are notfabricated as TFTs are connected with the substrate by a surfacemounting technology (SMT) such as a Chip on Glass (COG) process whichcomprises directly mounting the IC chips on a substrate and connectingthem with each of the connecting terminals, and a Tape Automated Bonding(TAB) process which comprises mounting each of the IC chips on aflexible support made of an organic resin, and then connecting the resinsupport with the display substrate.

It is further possible to fabricate the semiconductor layers of the TFTsat a lower temperature and yet to realize quick response TFTs having aconsiderably increased carrier mobility by taking advantage of, insteadof the conventional polycrystalline or amorphous semiconductors, asemi-amorphous semiconductor which is based on a novel concept.

The semi-amorphous semiconductor can be obtained by applying a heattreatment to crystallize a thin film having deposited by processes suchas low-pressure chemical vapor deposition (LPCVD), sputtering,plasma-assisted chemical vapor deposition (PCVD), and the like. Theprocess for fabricating a semi-amorphous semiconductor film is describedbelow referring to the sputtering process as an example.

In depositing a film by sputtering from a single crystal target of asilicon semiconductor under a mixed gas atmosphere comprising hydrogenand argon, the heavy argon atoms strike the surface of the silicontarget to release free silicon atoms. The free silicon atoms thusreleased from the target then travel to the substrate on which the filmis to be deposited, accompanied by clusters composed of several tens toseveral millions of silicon atoms. During their travel in the sputteringchamber, hydrogen atoms come to bond with the dangling bonds of thesilicon atoms located at the outer periphery of the clusters, and theseclusters comprising the silicon-hydrogen bonding are maintained to thesurface of the substrate to deposit as a relatively ordered region.Thus, a highly ordered film comprising a mixture of amorphous siliconand clusters accompanied by Si—H bondings on the periphery is realizedon the surface of the substrate. By further heat treating the depositedfilm in the temperature range of from 450 to 700° C. in a non-oxidizingatmosphere, the Si—H bondings on the outer periphery of the clustersreact with each other to yield Si—Si bondings. The Si—Si bondings thusobtained exert a strong attractive force to each other. At the sametime, however, the clusters in their highly ordered state aresusceptible to undergo phase transition to attain a more ordered state,i.e., a crystallized state. Thus, because the Si—Si bondings in theneighboring clusters attract each other, the resulting crystals sufferlattice distortion which can be observed by laser Raman spectroscopy asa peak deviated in position to the lower frequency side from the 520cm⁻¹ which corresponds to the peak of a single crystal of silicon.

Furthermore, since the Si—Si bondings between the neighboring clusterscause anchoring (connecting) effects, the energy band in each cluster iselectrically connected with that of the neighboring cluster through theanchored locations. Accordingly, since there is no grain boundaries asin the conventional polycrystalline silicon which function as a barrierto the carriers, a carrier mobility as high as in the range of from 10to 200 cm²/V·sec can be obtained. That is, the semi-amorphoussemiconductor defined above is apparently crystalline, however, ifviewed from the electrical properties, there can be assumed a statesubstantially free from grain boundaries. If the annealing of a siliconsemiconductor were to be effected at a temperature as high as 1000° C.or over, instead of a moderate annealing in the temperature range offrom 450 to 700° C. as referred hereinbefore, the crystallizationnaturally would induce crystal growth to precipitate oxygen and the likeat the grain boundaries and would develop a barrier. The resultingmaterial is then a polycrystalline material comprising single crystalsand grain boundaries.

In the semi-amorphous semiconductors, the carrier mobility increaseswith elevating degree of anchoring. To enhance carrier mobility and toallow the crystallization to take place at a temperature lower than 600°C., the oxygen content of the film should be controlled to 7×10¹⁹ cm⁻³or lower, preferably, to 1×10¹⁹ cm⁻³ or lower.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a liquid crystal display deviceaccording to an embodiment of the present invention, composed of an m×ncircuit structure;

FIG. 2 is an outer appearance of a liquid crystal display deviceaccording to an embodiment of the present invention;

FIGS. 3(a)-3(d) are a schematic representation illustrating thefabrication process of a TFT according to an embodiment of the presentinvention;

FIG. 4 is a schematic view of a peripheral circuit of a liquid crystaldisplay device according to an embodiment of the present invention;

FIGS. 5(a) and 5(b) are a schematic representation of a connection oftransistors of a peripheral circuit according to a liquid crystaldisplay device of an embodiment of the present invention;

FIG. 6 is a schematic representation illustrating the arrangement of thepixel portion of a liquid crystal display device according to anembodiment of the present invention;

FIG. 7 is a schematic representation of a liquid crystal display deviceaccording to another embodiment of the present invention;

FIG. 8 is a schematic view illustrating a liquid crystal display deviceaccording to another embodiment of the present invention, composed of anm×n circuit structure; and

FIG. 9 is a schematic representation of a liquid crystal display deviceaccording to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention is described in further detail below referring tonon-limiting examples.

EXAMPLE 1

Referring to FIG. 1, a liquid crystal display device having an m×ncircuit structure is described. In FIG. 2 is also given the outerappearance of the liquid crystal display device. In FIG. 1, a shiftregister circuit portion 1 (a means for supplying a signal to a wiringof the X direction) being connected to a wiring of the X direction issolely fabricated as TFTs 5 in a similar manner as an active deviceprovided to a pixel 6, and a peripheral circuit portion (a means forsupplying a signal to a wiring of the Y direction) connected to a wiringof the Y direction is provided as ICs 4 which is connected to thesubstrate by TAB. The thin film transistors provided in the pixel 6 asthe active device and the thin film transistors 5 constituting the shiftregister circuit portion 1 are provided on a single substrate.

The actual arrangement of the electrodes and the like corresponding tothis circuit structure is shown in FIG. 6. In FIG. 6, however, thestructure is simplified and shown in a 2×2 structure.

Referring to FIG. 3, the process for fabricating the TFTs of the liquidcrystal display device according to the present invention is explained.In FIG. 3(A), a silicon oxide film is deposited to a thickness of from1,000 to 3,000 Å as a blocking layer 51 by radio frequency (RF)magnetron sputtering on a glass substrate 50 made of an economical glasswhich resists to a temperature of 700° C. or less, e.g., about 600° C.The actual film deposition was carried out in a 100% oxygen atmosphereat a film deposition temperature of 15° C., at an output of from 400 to800 W, and a pressure of 0.5 Pa. The film deposition rate using a quartzor a single crystal silicon as the target was in the range of from 30 to100 Å/minute.

On the silicon oxide film thus obtained was further deposited a siliconfilm, e.g. an amorphous silicon film, by LPCVD (low-pressure chemicalvapor deposition), sputtering, or plasma-assisted CVD (PCVD).

In case of the LPCVD process, film deposition was conducted at atemperature lower than the crystallization temperature by 100 to 200°C., i.e., in the range of from 450 to 550° C., e.g., at 530° C., bysupplying disilane (Si₂H₆) or trisilane (Si₃H₈) to the CVD apparatus.The pressure inside the reaction chamber was controlled to be in therange of from 30 to 300 Pa. The film deposition rate was 50 to 250Å/minute. Furthermore, optionally boron may be supplied to 1×10¹⁵ to1×10¹⁸ atoms·cm⁻³ in the film as diborane during the film deposition tocontrol the threshold voltage (V_(th)) of the N-TFT and that of theP-TFT to be approximately the same.

In case of sputtering, film deposition was conducted using a singlecrystal silicon as the target in an argon atmosphere having addedtherein from 20 to 80% of hydrogen, e.g., in a mixed gas atmospherecontaining 20% of argon and 80% of hydrogen. The back pressure prior tosputtering was controlled to 1×10⁻⁵ Pa or lower. The film was depositedat a film deposition temperature of 150° C., a frequency of 13.56 MHz, asputter output of from 400 to 800 W, and a pressure of 0.5 Pa.

In case of the deposition of a silicon film by a PCVD process, thetemperature was maintained, e.g., at 300° C., and monosilane (SiH₄) ordisilane (Si₂H₆) was used as the reacting gas. A high frequency electricpower was applied at 13.56 MHz to the gas inside the PCVD apparatus toeffect the film deposition.

The films thus obtained by any of the foregoing processes preferablycontain oxygen at a concentration of 5×10²¹ cm⁻³ or lower. If the oxygenconcentration is too high, the film thus obtained would not crystallize.Accordingly, there would be required to elevate the thermal annealingtemperature or to take a longer time for the thermal annealing. Too lowan oxygen concentration, on the other hand, increases an off-state leakcurrent due to a backlighting. Accordingly, the preferred range of theoxygen concentration was set in the range of from 4×10¹⁹ to 4×10²¹atoms·cm⁻³. The hydrogen concentration was for example 4×10²²atoms·cm⁻³, which accounts for 1% by atomic with respect to the siliconconcentration of 4×10²² atoms·cm⁻³. To enhance crystallization of thesource and drain portions, oxygen concentration is 7×10¹⁹ atoms·cm⁻³ orless, preferably 1×10¹⁹ atoms·cm⁻³ or less and oxygen may be addedselectively by ion-implantation to the channel forming regions of theTFT which constitute the pixel, to such an amount to give aconcentration in the range of from 5×10²⁰ to 5×10²¹ atoms·cm⁻³. Since nolight was irradiated to the TFTs in the peripheral circuit, it waseffective to impart thereto a higher carrier mobility while reducing theoxygen concentration in order to realize a high frequency operation.

After the amorphous silicon film was deposited at a thickness of from500 to 5,000 Å, e.g., at a thickness of 1,500 Å, the amorphous siliconfilm was then heat-treated at a moderate temperature in the range offrom 450 to 700° C. for a duration of from 12 to 70 hours in anon-oxidizing atmosphere. More specifically, the film was maintained at600° C. under a hydrogen atmosphere. Since on the surface of thesubstrate was provided an amorphous silicon oxide layer under thesilicon film, the whole structure was uniformly annealed because therewas no nucleus present during the heat treatment.

The process can be explained as follows. That is, during the filmdeposition step, the film construction maintains the amorphous structureand the hydrogen is present as a mixture. The silicon film thenundergoes phase transition from the amorphous structure to a structurehaving a higher degree of ordering by the annealing, and partly developsa crystallized portion. Particularly, the region which attains arelatively high degree of ordering at the film deposition of silicontend to crystallize at this stage. However, the silicon bonding whichbonds the silicon atoms each other attracts a region to another. Thiseffect can be observed by a laser Raman spectroscopy as a peak shiftedto a lower frequency side as compared with the peak at 522 cm⁻¹ for asingle crystal silicon. The apparent grain size can be calculated by thehalf width as 50 to 500 Å, i.e., a size corresponding to that of amicrocrystal, but in fact, the film has a semi-amorphous structurecomprising a plurality of those highly crystalline regions yielding acluster structure, and the clusters are anchored to each other by thebonding between the silicon atoms.

The semi-amorphous film yields, as a result, a state in which no grainboundary (referred to hereinafter as GB) exists. Since the carriereasily moves between the clusters via the anchoring, a carrier mobilityfar higher than that of a polycrystalline silicon having a distinct GBcan be realized. More specifically, a hole mobility, μh, in the range offrom 10 to 200 cm²/V·sec and an electron mobility, μe, in the range offrom 15 to 300 cm²/V·sec, are achieved.

On the other hand, if a high temperature annealing in the temperaturerange of from 900 to 1200° C. were to be applied in the place of amoderate temperature annealing as described hereinabove, impuritiesundergo a solid phase growth from the nuclei and segregate in the film.This results in the high concentration of oxygen, carbon, nitrogen, andother impurities at the GB into a barrier. Thus, despite the highmobility within a single crystal, the carrier is interfered at itstransfer from a crystal to another by the barrier at the GB. Inpractice, it is quite difficult to attain a mobility higher than orequal to 10 cm²/V·sec with a polycrystalline silicon at the present.Thus, in the liquid crystal display device according to the presentinvention, a semi-amorphous or a semi-crystalline structured siliconsemiconductor is utilized.

Referring to FIG. 3(A), a process for fabricating a region 22 for aP-TFT and a region 13 for an N-TFT is described. The silicon film wasmasked with a first photomask <1>, and subjected to photo-etching toobtain the region 22 (having a channel width of 20 μm) for the P-TFT onthe right-hand side of the FIGURE and the region 13 for the N-TFT on theleft-hand side of the FIGURE.

On the resulting structure was deposited a silicon oxide film as thegate insulator film to a thickness of from 500 to 2,000 Å, e.g., to athickness of 1,000 Å. The conditions or the film deposition were thesame as those employed in depositing the silicon oxide film to give ablocking layer. Alternatively, a small amount of fluorine may be addedduring the film deposition to fix sodium ions.

Further on the gate insulator film was deposited a silicon film dopedwith phosphorus at a concentration of from 1×10²¹ to 5×10²¹ atoms·cm⁻³,or a multilayered film composed of said silicon film doped withphosphorus, having provided thereon a layer of molybdenum (Mo), tungsten(W), MoSi₂, or WSi₂. The resulting film was patterned using a secondphotomask <2> to obtain a structure as shown in FIG. 3(B). Then, a gateelectrode 55 for the P-TFT, and a gate electrode 56 for the NTFT wereestablished, for example, by depositing first a 0.2 μm thickphosphorus(P)-doped silicon and a 0.3 μm thick molybdenum layer thereon,at a channel length of 10 μm. Referring to FIG. 3(C), a photoresist 57was provided using a photomask <3>, and to a source 59 and a drain 58for the P-TFT were added boron by ion implantation at a dose of from1×10¹⁵ to 5×10¹⁵ cm⁻². Then, as shown in FIG. 3(D), a photoresist 61 wasprovided using a photomask <4>, and to a source 64 and a drain 62 forthe N-TFT was added phosphorus by ion implantation at a dose of from1×10¹⁵ to 5×10¹⁵ cm⁻². The processes above were carried out via a gateinsulator film 54. However, referring to FIG. 3(B), the silicon oxide onthe silicon film may be removed utilizing the gate electrodes 55 and 56as the masks, and then boron and phosphorus may be directly added to thesilicon film by ion implantation.

The structure thus obtained was re-annealed by heating at 600° C. for aduration of from 10 to 50 hours. The impurities in the source 59 anddrain 58 of the P-TFT and those in the source 64 and drain 62 of theN-TFT were activated to obtain P⁺ and N⁺ TFTs. Under the gate electrodes55 and 56 were provided channel forming regions 60 and 63 with asemi-amorphous semiconductor.

According to the process for fabricating a liquid crystal display deviceof the present invention as described hereinabove, a CTFT can beobtained in a self-aligned method without elevating the temperature to700° C. or higher. Thus, there is no need to use a substrate made of anexpensive material such as quartz, and therefore it can be seen that theprocess is suited for fabricating liquid crystal display devices oflarge pixels according to the present invention.

The thermal annealing in this example was conducted twice, i.e., in thesteps of fabricating the structures shown in FIGS. 3(A) and 3(D).However, the annealing at the step illustrated in FIG. 3(A) may beomitted depending to the desired device characteristics, and theannealing at the step shown in FIG. 3(D) can cover the total annealing.In this way it is possible to speed up the fabrication process.

Referring to FIG. 3(E), a silicon oxide film was deposited as aninterlayer insulator 65 by the sputtering process mentionedhereinbefore. The method of depositing the silicon oxide film is notrestricted to a sputtering method, and there may be employed LPCVD,photo CVD, or normal pressure CVD. The silicon oxide film was deposited,e.g., to a thickness of from 0.2 to 0.6 μm, and on it was perforated anopening 66 for the electrode using a photomask <5>. The structure wasthen wholly covered with aluminum by sputtering, and after providinglead portions 71 and 72, as well as contacts 67 and 68 using a photomask<6>, the surface thereof was coated with a smoothening film of anorganic resin 69 such as a transparent polyimide resin, and subjectedagain to perforation for the electrodes using a photomask <7>.

As shown in FIG. 3(F), two insulated gate field effect thin filmtransistors were established in a complementary structure. To the outputterminal thereof was provided an Indium Tin Oxide (ITO) film bysputtering to thereby connect the TFTs with one of the transparent pixelelectrodes of the liquid crystal display device. In this case, the ITOfilm was deposited in the temperature range from room temperature to150° C., and then finished by annealing in oxygen or in atmosphere inthe temperature range of from 200 to 400° C. The ITO film was thenetched using a photomask <8> to form an electrode 70. Thus was finallyobtained a structure comprising a P-TFT 22, an N-TFT 13, and anelectrode 70 made of a transparent electrically conductive film on asingle glass substrate 50. The resulting TFT yielded a mobility of 20cm²/V·sec and a V_(th) of −5.9 V for the P-TFT, and a mobility of 40cm²/V·sec and a V_(th) of 5.0 V for the N-TFT.

Referring to FIG. 6, the arrangement of electrodes and the like in thepixel portion of the liquid crystal display device is explained. AnN-TFT 13 is assembled at the crossing of a first scanning line 15 and adata line 21, and another pixel N-TFT is provided at the crossing of thefirst scanning line 15 with another data line 14. On the other hand, aP-TFT is assembled at the crossing of a second scanning line 18 and adata line 21. Further, to the neighboring crossing of another firstscanning line 16 and the data line 21 is provided another pixel N-TFT.Thus is accomplished a matrix structure using CTFTs. The N-TFT 13 isconnected to the first scanning line 15 via a contact at the inputterminal of the drain portion 64, and the gate portion 56 is connectedto the data line 21 established in a multilayered wiring structure. Theoutput terminal of the source portion 62 is connected to the pixelelectrode 17 via a contact.

The P-TFT 22, on the other hand, is connected to the second scanningline 18 by the input terminal of the drain portion 58 via a contact,while the gate portion 55 is connected to the data line 21, and theoutput terminal of the source portion 59 is connected to the pixelelectrode 17 via a contact in the same manner as in the N-TFT. In thismanner is a single pixel established between (inner side) a pair ofscanning lines 15 and 18, with a pixel 23 comprising a transparentelectrode and a CTFT pair. By extending this basic pixel structure infour directions, a 2×2 matrix can be scaled up to give a large pixelliquid crystal display device comprising a 640×480 or a 1280×960 matrix.

In FIGS. 4 and 5 are given the circuit diagrams for the peripheralcircuit of the X direction. FIG. 4 illustrates the block function of theperipheral circuit being connected to a single wiring, and FIG. 5 showsthe circuit diagram for connecting the transistor in the unit. FIG. 5(A)corresponds to the block 7 in FIG. 4, and FIG. 5(B) shows the circuitstructure of the TFT corresponding to the block 8 in FIG. 4. It can beseen therefrom that the peripheral circuit is provided as a CMOS(complementary MOS) structure comprising an N-TFT 13 and a P-TFT 22 bothfabricated in the same process as that employed for the switchingdevice.

The substrate thus accomplished for use in the liquid crystal displaydevice was laminated with the facing substrate by a known process, andan STN (super-twisted nematic) liquid crystal was injected between thesubstrates. The ICs 4 were used for the other part of the peripheralcircuit. The ICs 4 were connected with each of the wirings for the Ydirection on the substrate by a TAB process. Thus was completed a liquidcrystal display device according to the present invention.

In the embodiment explained in the Example, only the peripheral circuitfor the X direction was fabricated as TFTs following the same processused for the switching device, while leaving over the rest of theperipheral circuit as ICs having connected to the wirings of the Ydirection. The present invention, however, is not only limited to thisconstruction, and the portion which can be more easily fabricated intoTFTs can be selected depending on the yield and the problems related tothe process technology at the fabrication of the TFTs.

As described above, on the substrate a semiconductor film whichcomprises a semiconductor (amorphous silicon semiconductor in thisExample) was formed extending over a display area and a peripheralcircuit portion and subsequently the semiconductor (amorphous siliconsemiconductor in this Example) is annealed into semi-amorphous orsemi-crystal semiconductor. The semi-amorphous semiconductor yields amobility ten times as high as, or even higher than that of the TFT usinga conventional non-single crystal semiconductor. Thus, the TFT accordingto the present invention is well applicable to the peripheral circuitsin which a rapid response is required, without subjecting the TFTs inthe peripheral circuit portion to a special crystallization treatmentwhich was requisite in the conventional TFTs; as a result, the TFTs forthe peripheral circuit portion could be fabricated by the same processutilized for fabricating an active device of the display area, asdescribed above.

EXAMPLE 2

In FIG. 7 is given a schematic view of a liquid crystal display deviceaccording to another embodiment of the present invention. The basiccircuits and the like are the same as those employed in the liquidcrystal display device described in Example 1. Referring to FIG. 7, theperipheral circuit connected to the wirings for the Y direction isconstructed with ICs 4, and the ICs 4 are directly provided on thesubstrate by a COG method.

The pad electrodes of the ICs 4 can be connected with the wirings of theY direction at a narrower interval by the use of a COG method instead ofthe TAB method. Thus, the process of the present example enables a finerdisplay pixel design. Furthermore, since the ICs are provided on thesubstrate, the total volume of the display device remains almostunchanged to give a thinner liquid crystal display device.

EXAMPLE 3

Referring to FIG. 8, a liquid crystal display device having an m×ncircuit structure is described below. Among the peripheral circuitportions (a means for supplying a signal to wirings of the X direction)connected to the wirings of the X direction, only an analog switch arraycircuit portion 1 is fabricated as TFTs in the same process as that usedfor fabricating the active device provided to a pixel 6. Furthermore,only an analog switch array circuit portion 2 in the peripheral circuitportion (a means for supplying a signal to wirings of the Y direction)connected to the wirings of the Y direction is also fabricated as TFTs,while providing the rest of the peripheral circuit portion by makingthem into ICs 4 having connected to a substrate using a COG method. Theperipheral circuit portion provided as TFTs is in a CTFT structure(complementary structure) similar to the active device established tothe pixels.

In FIG. 6 is provided the actual arrangement of the electrodes and thelike corresponding to the circuit structure described above. However,the portion shown in FIG. 6 corresponds to a 2×2 matrix for the sake ofsimplicity.

Electrodes, TFTs, wirings, and the like were formed on a singlesubstrate in the same fabrication process as that used in Example 1. Inthis process, the switching device (e.g. TFTs 13 and 22 in FIG. 6) and apart of the peripheral circuits were fabricated in TFTs on the samesubstrate. This substrate was laminated with the facing substrate and anSTN liquid crystal was injected between the substrates in the samemanner as in Example 1. The remaining peripheral circuit was establishedas ICs 4. The ICs 4 were connected with each of the wirings of the X andY directions by a COG method. To the ICs 4 was connected each of theconnection leads for power supply and data input, but there was no FPCattached all over one edge of the substrate for the connection. Thereliability of the liquid crystal display device was increased by thusreducing the number of connections.

In the liquid crystal display device described in the present Example,only the analog switch array circuit portion 1 among the peripheralcircuit of the X direction and the analog switch array circuit portion 2among the peripheral circuit of the Y direction were fabricated intocomplementary TFTs in the same process as that used for fabricating aswitching device. The remaining peripheral circuit portion was providedas the ICs 4. However, a liquid crystal display device is not restrictedto this structure and the portion which can be more easily fabricated asTFTs can be selected and fabricated as TFTs, depending on the yield andthe problems related to the process technology at the fabricationthereof. In an embodiment according to the present invention as referredin this Example, a semi-amorphous semiconductor was used for thesemiconductor film. It yields a mobility ten times or more higher thanthat of the TFT using a conventional non-single crystal semiconductor.Thus, the TFT according to the present invention is well applicable toperipheral circuits in which a rapid response is required, withoutsubjecting the TFTs in the peripheral circuit portion to a specialcrystallization treatment which was requisite in the conventional TFTs;as a result, the TFTs for the peripheral circuit portion could befabricated by the same process utilized for fabricating an activedevice.

Furthermore, since a CTFT structure is employed for the active matrixbeing connected to the liquid crystal pixels, the operational margin wasincreased and a constant display level could be maintained withoutfluctuation in the pixel voltage. Even when malfunctioning occurred onone of the TFTs, it was possible to maintain the display withoutsuffering a perceptible defect.

EXAMPLE 4

Referring to a schematic diagram in FIG. 9, a liquid crystal displaydevice according to another embodiment of the present invention isdescribed below. The basic circuits and the like are the same as thoseof the liquid crystal display device described in Example 3. Among theperipheral circuits connected to the wirings of the Y direction in FIG.9, the peripheral circuit portion composed of ICs 4 comprises ICs havingdirectly established on the substrate by a COG method. The ICs 4 aredivided into two portions, and one of them is provided at the upperportion of the substrate and the other is provided at the lower portionof the substrate. By employing such an arrangement, the pad electrodesof the ICs 4 and the wirings of the Y direction can be connected at anarrower interval as compared with the case in which the ICs areprovided only on one side of the substrate. This structure can betherefore characterized by a finer display pixel design. Furthermore,since the ICs are directly bonded on the substrate, the structure can berealized with a negligible change in volume and hence enables a thinnerliquid crystal display device.

In the examples described above, the TFTs of the active device were eachfabricated into a CMOS structure. However, this structure is notrequisite and modifications such as constructing the structure with onlyN-TFTs or with only P-TFTs are also acceptable. In these cases, however,the number of the devices in the peripheral circuit would be increased.

Furthermore, the position at which the TFTs are established can bevaried. That is, the TFT need not be provided only to one side, i.e.,only to either of the sides connected to the wirings of the X directionand the Y direction, but there may be also provided a second TFT on theother side to connect the TFTs in turns to halve the TFT density. Such astructure realizes an increase in the production yield.

The present invention provides a fine and precise liquid crystal displayby overcoming the problems ascribed to the limitations related to thetechnological difficulty in the peripheral connections. Furthermore, thereliability on the connections is improved, since unnecessaryconnections between the outer peripheral circuits and the wirings alongthe X and Y directions are minimized.

The area occupation of the display substrate itself is reduced, sinceonly a part of the peripheral circuits are fabricated into a TFT. Italso allows a more freely designed substrate having a shape anddimension according to the requirements. Further it is possible toreduce the production cost, since the problems related to the productionof the TFTs can be avoided while making those portions having a higherproduction yield into TFTs.

The use of a semi-amorphous semiconductor as the semiconductor film ofthe TFTs enables a rapid response, making the TFTs well applicable toperipheral circuits. Thus, the TFTs for the peripheral circuits arereadily fabricated simultaneously utilizing the fabrication process forthe active devices without any additional special treatments.

The liquid crystal display device according to the present inventionprovides a stable display because it operates without floating theliquid crystal potential. Furthermore, since the drive capacity of theCTFTs which function as the active devices is high, the operationalmargin is increased. Since the peripheral driving circuit can be furthersimplified, it results in a more compact display device which isproduced at a lower production cost. A high driving capacity can berealized with three signal lines and counter electrodes using a verysimple signal. It is also possible to increase the switching rate. Evenwhen a part of the TFTs should get out of order, a compensation functionworks to a certain degree. Since the carrier mobility increases to 10times or more than that of a TFT using an amorphous silicon, the size ofthe TFT can be minimized and two TFTs can be connected to a single pixelwithout decreasing the aperture ratio. Furthermore, the advantagesenumerated above can be realized by only using twice more the photomasksas compared with a conventional process in which only N-TFTs are used.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

1. A display device comprising; a substrate; a plurality of first signallines formed over said substrate; a plurality of second signal linesextending across said first signal lines over said substrate; aplurality of switching elements formed at each intersection of saidfirst and second signal lines, each of said switching elementscomprising at least one first thin film transistor wherein a gateelectrode of said first thin film transistor is electrically connectedto one of said plurality of first signal lines and an impurity region ofsaid first thin film transistor is electrically connected to one of saidplurality of second signal lines; a smoothing film comprising an organicresin formed over said switching elements; a plurality of pixelelectrodes formed over said smoothing film and electrically connected tosaid switching elements through contact holes formed in said smoothingfilm; and a first driver circuit comprising at least one IC chipelectrically connected to said plurality of second signal lines; and asecond driver circuit electrically connected to said plurality of firstsignal lines, wherein said second driver circuit comprises second thinfilm transistors formed over said substrate.
 2. The display deviceaccording to claim 1 wherein said pixel electrodes comprise indium tinoxide.
 3. The display device according to claim 1 wherein at least oneof said first and second thin film transistors has a channel regioncomprising semi-amorphous silicon.
 4. The display device according toclaim 1 wherein each of said first and second thin film transistors hasa top-gate structure.
 5. The display device according to claim 1 whereinsaid smoothing film comprises polyimide.
 6. A display device comprising:a substrate; a plurality of first signal lines formed over saidsubstrate; a plurality of second signal lines extending across saidfirst signal lines over said substrate; a plurality of switchingelements formed at each intersection of said first and second signallines, each of said switching elements comprising at least one firstthin film transistor, wherein a gate electrode ol said first thin filmtransistor is electrically connected to one of said plurality of firstsignal hues and an impurity region of said first thin film transistor iselectrically connected to one of said plurality of second signal lines;a smoothing film comrising an organic resin fonned over said switchingelements; a plurality of pixel electrodes formed over said smoothingfilm and electrically connected to said switching elements throughcontact holes formed in said smoothing film; and a first driver circuitcomprising at least one IC chip electrically connected to said pluralityof second signal lines; and a second driver circuit electricallyconnected to said plurality of first signal lines, wherein said seconddriver circuit comprises second thin film transistors formed over saidsubstrate, wherein said first driver circuit is connected to saidsubstrate through a tape automated bonding process.
 7. The displaydevice according to claim 6 wherein said IC chip is mounted on aflexible support.
 8. The display device according to claim 6 whereinsaid pixel electrodes comprise indium tin oxide.
 9. The display deviceaccording to claim 6 wherein at least one of said first and second thinfilm transistors has a channel region comprising semi-amorphous silicon.10. The display device according to claim 6 wherein each of said firstand second thin film transistors has a top-gate structure.
 11. A displaydevice comprising: a substrate; a plurality of first signal lines formedover said substrate; a plurality of second signal lines extending acrosssaid first signal lines over said substrate; a plurality of switchingelements formed at each intersection of said first and second signallines, each of said switching elements comprising at least one thin filmtransistor; a smoothing film comprising an organic resin formed oversaid switching elements; a plurality of pixel electrodes formed oversaid smoothing film and electrically connected to said switchingelements through contact holes formed in said smoothing film; and adriver circuit comprising at least one IC chip for driving saidswitching elements, wherein said IC chip is formed on a flexible supportand said flexible support is connected to said substrate.
 12. Thedisplay device according to claim 11 wherein said thin film transistorhas a top-gate structure.
 13. The display device according to claim 11wherein said pixel electrodes comprise indium tin oxide.
 14. The displaydevice according to claim 11 wherein said IC chip is electricallyconnected to either one of said first signal lines or said second signallines.
 15. The display device according to claim 11 wherein said drivercircuit further comprises thin film transistors formed over saidsubstrate.
 16. The display device according to claim 11 wherein saidthin film transistor has a channel region comprising semi-amorphoussilicon.
 17. A display device comprising: a substrate; a plurality offirst signal lines formed over said substrate; a plurality of secondsignal lines extending across said first signal lines over saidsubstrate; a plurality of switching elements formed at each intersectionof said first and second signal lines, each of said switching elementscomprising at least one first thin film transistor, wherein a gateelectrode of said first thin film transistor is electrically connectedto one of said plurality of first signal lines and an impurity region ofsaid first thin film transistor is electrically connected to one of saidplurality of second signal lines; a smoothing film comprising an organicresin formed over said switching elements; a plurality of pixelelectrodes formed over said smoothing film and electrically connected tosaid switching elements through contact holes formed in said smoothingfilm; and a first driver circuit comprising at least one IC chipelectrically connected to said plurality of second signal lines; and asecond driver circuit electrically connected to said plurality of firstsignal lines wherein said second driver circuit comprises second thinfilm transistors formed over said substrate, wherein said IC chip ismounted over said substrate.
 18. The display device according to claim17 wherein each of said first and second thin film transistors has atop-gate structure.
 19. The display device according to claim 17 whereinsaid pixel electrode comprise indium tin oxide.
 20. The display deviceaccording to claim 17 wherein at least one of said first and second thinfilm transistors has a channel region comprising semi-amorphous silicon.21. A display device comprising: a substrate; a plurality of firstsignal lines formed over said substrate; a plurality of second signallines extending across said first signal lines over said substrate; aplurality of switching elements formed at each intersection of saidfirst and second signal lines, each of said switchihg elementscomprising at least one first thin film transistor wherein a gateelectrode of said first thin film transistor is electrically connectedto one of said plurality of first signal lines and an impurity region ofsaid first thin film transistor is electrically connected to one of saidplurality of second signal lines; an organic resin film formed over saidswitching elements; a plurality of pixel electrodes formed over saidorganic resin film and electrically connected to said switching elementsthrough contact holes formed in said organic resin film; and a firstdriver circuit comprising at least one IC chip electrically connected tosaid plurality of second signal lines; and a second driver circuitelectrically connected to said plurality of first signal lines, whereinsaid second driver circuit comprises second thin film transistors formedover said substrate.
 22. The display device according to claim 21wherein said organic resin film comprises polyimide.
 23. The displaydevice according to claim 21 wherein each of said first and second thinfilm transistors has a top-gate structure.
 24. A display devicecomprising: a substrate; a plurality of first signal lines formed oversaid substrate; a plurality of second signal lines extending across saidfirst signal lines over said substrate; a plurality of switchingelements formed at each intersection of said first and second signallines, each of said switching elements comprising at least one thin filmtransistor; an organic resin film formed over said switching elements; aplurality of pixel electrodes formed over said organic resin film andelectrically connected to said switching elements through contact holesformed in said organic resin film; and a driver circuit comprising atleast one IC chip for driving said switching elements, wherein said ICclip is formed on a flexible support and said flexible support isconnected to said substrate.
 25. The display device according to claim24 wherein said organic resin film comprises polyimide.
 26. The displaydevice according to claim 24 wherein said driver circuit includes afirst driver circuit including said IC chip and electrically connectedto said plurality of second signal lines and a seconds driver circuitcomprising second thin film transistors and electrically connected tosaid plurality of first signal lines.
 27. A display device comprising: asubstrate; a plurality of first signal lines formed over said substrate;a plurality of second signal lines extending across said first linesover said substrate; a plurality of switching elements formed at eachintersection of said first and second signal lines, each of saidswitching elements comprising at least one first thin film transistor,wherein a gate electrode of said first thin film transistor iselectrically connected to one of said plurality of fitst signal linesand an impurity region of said first thin film transistor iselectrically connected to one of said plurality of second signal lines;an organic resin film formed over said switching elements; a pluralityof pixel electrodes formed over said organic resin film and electricallyconnected to said switching elements through contact holes formed insaid organic resin film; and a first driver circuit cornpnsing at leastone IC chip electrically connected to said plurality of second signallines; and a second driver circuit electrically connected to saidplurality of first signal lines wherein said second driver circuitcomprises second thin film transistors formed over said substrate,wherein said IC chip is mounted over said substrate.
 28. The displaydevice according to claim 27 wherein said organic resin film comprisespolyimide.
 29. The display device according to cIaim 27 wherein each ofsaid first and second thin film transistors has a top-gate structure.30. A display device comprising: a substrate; a plurality of firstsignal lines formed over said substrate; a plurality of second signallines extending across said first signal lines over said substrate; aplurality of switching elements formed at each intersection of saidfirst and second signal lines, each of said switching elementscomprising at least one first thin film transistor wherein a gateelectrode of said first thin film transistor is electrically connectedto one of said plurality of first signal lines and an impurity region ofsaid first thin film transistor is electrically connected to one of saidplurality of second signal lines; a smoothing film formed over saidswitching elements; a plurality of pixel electrodes formed over saidsmoothing film and electrically connected to said switching elementsthrough contact holes formed in said smoothing film; and a first drivercircuit comprising at least one IC chip electrically connected to saidplurality of second signal lines; and a second driver circuitelectrically connected to said plurality of first signal lines, whereinsaid second driver circuit comprises second thin film transistors formedover said substrate.
 31. The display device according to claim 30wherein each of said first and second thin film transistors has atop-gate structure.
 32. A display device comprising: a substrate; aplurality of first signal lines formed over said substrate; a pluralityof second signal lines extending across said first signal lines oversaid substrate; a plurality of switching elements formed at eachintersection of said first and second signal lines, each of saidswitching elements comprising at least one thin film transistor; asmoothing film formed over said switching elements; a plurality of pixelelectrodes formed over said smoothing film and electrically connected tosaid switching element through contact holes formed in said smoothingfilm; and a driver circuit comprising at least one IC chip for drivingsaid switching elements, wherein said IC chip is formed on a flexiblesupport and said flexible support is connected to said substrate. 33.The display device according to claim 32 wherein said driver circuitincludes a first driver circuit including said IC chip and electricallyconnected to said plurality of second signal lines and a second drivercircuit comprising second thin film transistors and electricallyconnected to said plurality of first signal lines.
 34. A display devicecomprising: a substrate; a plurality of first signal lines formed oversaid substrate; a plurality of second signal lines extending across saidfirst signal lines over said substrate; a plurality of switchingelements formed at each intersection of said first and second signallines, each of said switching elements comprising at least one firstthin film transistor, wherein a gate electrode or said first thin filmtransistor is electrically connected to one of said plurality of firstsignal lines and an impurity region of said first thin film transistoris electrically connected to one of said plurality of second signallines; a smoothing film formed over said switching elements; a pluralityof pixel el&trodes formed over said smoothing film and electricallyconnected to said switching elements through contact holes formed insaid smoothing film; and a first driver circuit comprising at least oneIC chip electrically connected to said plurality of second signal lines;and a second driver circuit electrically connected to said plurality offirst signal lines wherein said second driver circuit comprises secondthin film transistors formed over said substrate, wherein said IC chipis mounted over said substrate.
 35. The display device according toclaim 11 wherein said driver circuit includes a first driver circuitincluding said IC chip and electrically connected to said plurality ofsecond signal lines and a second driver circuit comprising second thinfilm transistors and electrically connected to said plurality of firstsignal lines.
 36. The display device according to claim 1 wherein eachof said first and second driver circuits comprises a shift registercircuit.
 37. The display device according to claim 6 wherein each ofsaid first and second driver circuits comprises a shift registercircuit.
 38. The display device according to claim 17 wherein each ofsaid first and second driver circuits comprises a shift registercircuit.
 39. The display device according to claim 21 wherein each ofthe first and second driver circuits comprises a shift register circuit.40. The display device according to claim 27 wherein each of the firstand second driver circuits comprises a shift register circuit.
 41. Thedisplay according to claim 30 wherein each of the first and seconddriver circuits comprises a shift register circuit.
 42. The displaydevice according to claim 34 wherein each of the first and second drivercircuits comprises a shift register circuit.
 43. The display deviceaccording to claim 1 wherein each of said first and second comprises alatch circuit.
 44. The display device according to claim 6 wherein saidfirst driver circuit comprises a latch circuit.
 45. The display deviceaccording to claim 17 wherein said first driver circuit comprises alatch circuit.
 46. The display device according to claim 21 wherein saidfirst driver circuit comprises a latch circuit.
 47. The display deviceaccording to claim 27 wherein said first driver circuit comprises alatch circuit.
 48. The display device according to claim 30 wherein saidfirst driver circuit comprises a latch circuit.
 49. The display deviceaccording to claim 34 wherein said first driver circuit comprises alatch circuit.